Exam 3: RR (4)

i.as it enters the __, the nascent polypeptide is acted on by __located within either the membrane or __of the RER

1. __ portion containing the signal peptide is removed from most nascent polypeptides by a __, the ssignal peptidase

RER cistern

enzymes

lumen

N-terminal

proteolytic enzyme

a.__are added to the nascent protein by the enzyme __

i.both the __ ande __ are integral membrane proteins associated with the __that act on nascent proteins as they enter the __

carbs

oligosaccharyltransferase

signal peptidase and oligosaccharyltransferase

translocon

ER lumen

i.the RER is a major protein processing plant and is packed with molecular __that recognize and bind to __ or __ and give them the opportunity to attain their correct 3D structure

1.the Er lumen also contains a number of __, such as __

chaperones

unfolded or misfolded proteins

protein-processing enzymes

protein disulfide isoemrase

i.proteins enter the ER lumen with their __ in the __state, but they leave the compartment with may of these residues joined to one another as __

1.the formation and rearrangement of __ bonds is catalyzed by __

cysteine residues

reduced

oxidized disulfides

disulfide

PDI

a.disulfide bonds play an important role in __ that are present at the extraceullular surface of the plasma membrane or secreted into the extracellular space

maintaining the stability of proteins

i.the ER is constructed for its role as a __ for the __ of the cell

1.its membrane provides a __ to which many __can attach

a.the __of the __ provides a specialized local environment that favors the __, __, and __ of a selected subset of the cell’s proteins

port of entry

biosynthetic pathway

large surface area

ribosomes

lumen

ER cisternae

modification, folding, and assembly

1.the segregation of these newly synthesized proteins in the __ removes them from the cytosol and allows them to be __ and __ toward their ultimate destination, whether outside the cell or within one of the cytoplasm’s membranous organelles

ER cisternae

modified and dispatched

a.Synthesis of Integral Membrane Proteins on Membrane-Bound Ribsomes

i.integral membrane proteins—other than those of the mitochondria and chloroplasts—are also synthesized on __

1.these membrane proteins are translocated into the __ as they are synthesized using the same machinery described for the synthesis of secretory and lysosomal proteins

membrane-bound ribosomes of the ER

ER membrane

a.unlike __ and __, which pass entirely through the ER membrane during __, __ contain one or more __ that are shunted directly from the channel of the __into the lipid bilayer

soluble secretory and lysosomal proteins

translocation

integral proteins

hydrophobic transmembrane segments

translocon

i.as a polypeptide passes through the __, it is proposed that this __ in the channel continually opens and closes, which gives each segment of the nascent polypeptide an opportunity to __itself according to its __properties into either the aqueous compartment within the __ or the surrounding __ of the lipid bilayer.

translocon

lateral “gate”

partition

solubility

translocon channel

hydrophobic core

1.those segments of the nascent polypeptide that are __will spontaneously “__” into the lipid bilayer and become __ of an integral membrane protein

a.this concept has received strong support from an in vitro study in which __were given the opportunity to translocate custom-designed nascent proteins that contained test segments of varying __

hydrophobic

dissolve

transmembrane segments

translocons

hydrophobicity

The more hydrophobic the test segment, what?

the greater the likelihood it would pass through the wall of the translocon and become integrated as a transmembrane segment of the bilayer

i.__ membrane proteins can have an orientation with their __ facing either the cytosol or the lumen of the ER

1.the most common determinant of membrane protein alignment is the presence of __ flanking the __end of a __segment

single-spanning

N-terminus

positively charged amino acid residues

cytosolic

transmembrane

1.during the synthesis of membrane proteins, the inner lining of the __is thought to orient the nascent polypeptide so that the more __end faces the __

1.once determined, every other transmembrane segment has to rotated __before it can exit the translocon

a.suggestion: a translocon, by itself, is capable of what?

180o

properly orienting transmembrane segments

i.it is more than a simple passageway through the ER membrane; it is a complex machine capable of __and performing complex mechanical activities

recognizing various signal sequences

a.Membrane Biosynthesis in the ER

i.membranes don’t arise as __, but from __ that grow as new proteins and lipids are inserted

ii.membrane components move from the __to virtually every other compartment in the cell

new entities from pools of protein and lipid that mix together

preexisting membranes

ER

1.as the membrane moves from one compartment to the next, its proteins and lipids are __

a.these modifications contribute to giving each membrane compartment a __ and __

ii.Cellular membranes are __: the two phospholipid layers of a membrane have different compositions

modified by enzymes that reside in the cell’s various organelles

unique composition and distinct identity

asymmetric

1.the asymmetry is established by the __ and maintained as __

a.domains situated at the __surface of the ER membrane can be identified on the __ surface of __, the __surface of __, and the __ surface of the __

ER

membrane carriers bud from one compartment and fuse to the next

cytosolic

cytosolic

transport vesicles

cytosolic

Golgi cisternae

internal

plasma membrane

i.domains situated at the __ of the ER membrane maintain their __and are found at the __ of the plasma membrane

1.including its high __and abundance of proteins with __ and __, the lumen of the ER is a lot like the __

luminal surface

orientation

external surface

calcium concentration

disulfide bonds

carb chains

extracellular space

i.Most __ are synthesized completely within the ER except for __ (synthesis begins in __and is completed in __) and some unique __ of the __ and __ membranes, which are synthesized by enzymes that reside in those membranes

1.the enzymes involved in the synthesis of __ are integral proteins of the ER memrbaen with their __ facing the __

membrane lipids

sphingomyelin and glycolipids

ER

Golgi

lipids

mitochondrial and chloroplast membranes

phospholipids

active sites

cytosol

a.newly synthesized phospholipids are inserted into the __of the bilayer facing the cytosol

i.some of these are __ into the opposite leaflet through the action of enzymes called __

lipids are carried from the __ to __ and __ as part of the __ that makes up the walls of __

half

flipped

flippases

Er to Golgi and plasma membrane

bilayer

transport vesicles

i.membranes of different organelles have markedly different __composition, which indicates that what happens?

lipid

changes take place as membrane flows through the cell

several factors may contribute to such changes
The first?

a.meost membranous organelles contain enzymes that modify lipids already present within a membrane, converting one type of phospholipid to another

several factors may contribute to such changes
The second?

a.when vesicles bud from a compartment, some types of phospholipids may be preferentially included within the membrane of the forming vesicle, while other types may be left behind

several factors may contribute to such changes
The third?

a.cells contain lipid-transfer proteins that can bind and transport lipids through the aqueous cytosol from one membrane compartment to another

i.these proteins facilitate the movement of specific __from the __to other organelles without the involvement of __

1.__ is thought to occur at sites where the ER comes into __ to the outer membrane of other organelles